A purpose of a propeller is to convert the rotary motion of an engine into propulsive force, or thrust.
Increasingly, the design of modern propeller arrangements is influenced by environmental considerations. Such considerations include a need for increased fuel efficiency and reduced noise generation.
It is known that the efficiency of a traditional single-propeller assembly is increased by the provision of a second propeller assembly rotating in an opposite direction downstream of the first propeller. However, such arrangements, known in the art as contra-rotating propellers, generally suffer from increased noise generation compared to single propeller assembly arrangements.
It is understood that the increased noise generation is partly due to stream tube contraction and interaction between the second propeller blades and the vortices formed at the tips of the first propeller blades. The tip vortices' position is in line with the stream tube, and follows its contraction from the plane of first row of propellers to the plane of the second row of propellers. However, the amount of stream tube contraction varies, depending on the flight condition, the contraction tending to be greater in higher thrust conditions such as take off.
If the diameter of the second propeller assembly is larger than the diameter of the stream tube then noise generation will increase as the second row of propellers chop through the vortices. On the other hand, if the second propeller assembly diameter is reduced too far, with the aim of having the second propellers' tips radially inward from the vortices at all flight conditions, the contra-rotating system can loose propulsive efficiency.
US 2010/0047068 proposes a contra-rotating propeller engine in which the diameter of the rear propellers is less than the diameter of the front propellers.
WO05/017351 and WO08/8142498 propose telescopic rotor blades in which the rotor blades are split into sections that can move radially relative to each other to vary the overall lengths of the blades. EP A 2009246 proposes a propeller blade pitch control mechanism in which turning of the blade results in a slight outward movement of the blade.